This invention relates to a signal level instrument which is useful in accurately determining the level or amplitude of various R.F. signals. There are a number of uses for such instruments, the most prevalent of which is in connection with the testing and alignment of cable television signals. A technician must verify the level of signal present at various points in the cable system, for example at all the terminals, junctions, etc., and due to this he often must carry a portable instrument up a pole to some connection point, attach a probe or lead of the instrument, and then use the instrument to check the signal level on all of the various channels available in the system.
At present the Federal Communications Commission requires 20 channel capability of cable television systems, thus the technician may have to check the signal level in that many different channels, or more, depending upon the capacity of the system. In addition, these systems utilize a low band pilot signal, typically around 75 MHz, and a high band pilot signal which is typically in the order of 180 to 300 MHz to keep the system balanced. The pilot signals provide fixed references against which the trunk line amplifiers can automatically adjust to maintain an essentially constant level over a wide range of temperature conditions. These pilot signals must also be checked for signal level.
In different cable television systems, some channels may be provided below the lowest VHF channel (channel 2), on which additional television and/or communication signals can be provided. These are commonly referred to as the "sub-low" channels. They are also sometimes used for two-way communication within the system, for example where local live television signals may be originated in the system's area, transmitted on one of the sub-low channels to the antenna and processor, and then converted to be transmitted back into the system on one of the unused VHF channels.
In addition, a number of "mid-band" channels are available within the VHF spectrum, between channels 6 and 7 together with so-called "super-band" channels above the frequency of VHF channel 13. Both the mid-band and superband channels can be used for video or data transmission within the system. By way of example, a subscription type adapter can be used to receive and decode special programs such as movies transmitted on these channels.
In all, a signal level instrument may be required to have a total band width encompassing some 36 different channels, and it may be necessary for the technician to check individually the signal level on a large percentage of these. Present instruments used for this purpose provide a calibrated meter from which the technician must read a signal level value, usually in the range of 0-20 dBmV, which must be subtracted from the attenuation setting manually preset into the instrument through a bandswitch. This value must be compensation, by use of a chart from which a compensation adjustment is set into the instrument, depending upon the frequency of the instrument, depending upon the frequency of the signal being tested. Thus, when the technician must perform these operations, with the presently used instruments, while on a pole, and make the necessary adjustments, recordings, calculations, etc., there is considerable chance for error to be introduced in the recorded values. Furthermore, although gain compensation may be built into such instruments, it has been common practice to compromise the gain compensation over the range of the instrument, in which case the compensation may not be precisely accurate for any given channel.